Agricultural machines such as forage harvesters are generally provided with metal detectors for detecting the presence of metal objects in crop material picked up from a field. Upon detection of a metal object the metal detector produces an output signal to stop the crop feed mechanism before the metal object can reach the cutter knives and cause damage. As shown in FIG. 2, the metal detector is frequently located within a housing 70 that is in turn located within a rotatable lower front feed roll 72. Crop material 73 is picked up from a field by a pick-up mechanism (not shown) and fed between lower and upper front feed rolls 72, 71 and lower and upper rear feed rolls 76, 74 to a cutter mechanism comprising a rotating reel 78 having peripheral cutter knives 75 cooperating with a stationary cutter bar 77 to cut the crop material. Obviously, metal objects fed between knives 75 and cutter bar 76 can severely damage the cutter mechanism. The metal detector prevents such damage by sensing metal objects and, upon sensing such an object, producing an output signal which is applied to a stop mechanism 98 to stop the feed rolls.
Because the metal detector coils are located within housing 70 and also within the feed roll 72, they are difficult to access. Furthermore, the coils are usually encased in a potting material and the electronic detection circuits to which the outputs of the coils are connected are also enclosed within housing 70 as described in U.S. Pat. No. 4,433,528, thereby making access to coil test points even more difficult. On the other hand, the resistance of the coils should be checked because changes in the resistance of a coil affects the output signal from the coil and thus the sensitivity of the metal detector to metal objects passing in proximity to the coil.